I think having all this state stored on the component instance will lead to a lot of possible bugs. e.g. if the component is rendered with one @scope, and then the @scope value changes, then the getter will be automatically re-run. But there is nothing to clear all this internal state, so we'll end up with duplicate data in these sets/arrays.

As a general rule, we should aim to avoid any kind of side-effects in getters.

I think I'd aim to build everything you need inside the getter, and then return it. So something like:

get textAndPlaceholders(){
  ...
  return {
    parts: ...,
    elements: ...,
    ...
  }
}

And then

<template>
  {{#let this.textAndPlaceholders as |infos|}}
    {{#each infos.parts as |part|}}
      {{part}}
    {{/each}}

It's probably worth having a test for that case where the scope and/or opts arguments change at runtime.